I am attempting to replace the valve supplying water to the toilet water tank, but am unable to unscrew it. It turns, but it does not come off. What can I do? Need help ASAP! Thank you!

If the stop looks like this,(see image) then it's a compression stop. Hold the valve with one set of pliers while unscrewing the nut counterclockwise with another set. When replacing the angle stop leave the big and small compression nuts on the pipes and just attach the new angle stop to the old compression nuts. Good luck, Tom